The pneumatic system of an aircraft comprises a bleed air distribution system that can be fed by various sources in order to provide temperature-controlled air at high pressure for various purposes, such as, for example, among other things for supplying the air conditioning system of the aircraft.
During a flight the engines or an auxiliary power unit are/is used as sources/a source of bleed air. On the ground, the aircraft can also be supplied by way of an external bleed air source that is connected from the outside to the high-pressure ground connection of the bleed air supply system. The pre-conditioned bleed air in the distribution system usually has a pressure of approximately 4 bar and a temperature ranging between 150° C. and 260° C.
An overheat detection system OHDS monitors the actual temperatures in the vicinity of the hot-air lines of the bleed air system. The overheat detection system is used to prevent damage to the aircraft body and its components, which damage can arise due to a leak or fracture in a hot-air line of the bleed air system. Since the hot-air lines are located in the region of the engine suspension system, in the wings, in the fuselage and in the lower fairing, a leak can quickly result in impeding vital systems and structures. Within the context of this description no differentiation is made between a line leak and a line fracture, since they differ only in the types of openings in the line but not in their effect on the operation of the aircraft. The terms line leak and line fracture are therefore used synonymously in this document.
For the purpose of monitoring the bleed air supply system, the overheat detection system comprises sensor devices that are controlled by way of discrete lines and a data bus. The sensor device of the overheat detection system is arranged parallel to the bleed air lines and is used for measuring the temperature in the vicinity of these supply lines. Determining the temperature of the surroundings takes place in the form of impedance measuring by means of a measuring loop. In this arrangement a measuring loop usually comprises several coaxial sensors, connected in series, which sensors map the so-called OHDS loop.
In the case of a leak detected in the bleed air system of the aircraft, the affected region of the system is isolated and any further supply of bleed air from the associated bleed air source is stopped. Automatic isolation of the region affected by the line leak is furthermore accompanied by a visual or an acoustic warning signal in the cockpit. In this arrangement, for the purpose of accelerating the now required maintenance work, the position of the leak is displayed on the onboard maintenance system of the aircraft at a specified accuracy.
Impeccable automatic operation of the overheat detection system requires the electrical supply in the aircraft to be active, and electrically controllable valves to be present. As a rule, this condition is met both in flight and during time on the ground so that in the case of a leak the supply of further bleed air from a bleed air source such as the engine bleed air system or the auxiliary power unit can be stopped. However, if the bleed air supply is fed from an external bleed air source by way of the high-pressure ground connection of the bleed air supply system, then said bleed air supply cannot be automatically stopped because the high-pressure ground connection is designed as a nonreturn valve.